The invention relates to a PFBC power plant with a combustor operating at a pressure exceeding the atmospheric pressure and in which combustion gases drive a gas turbine which drives a compressor compressing combustion air. The walls of the combustor are water-cooled and form at least part of a feed water preheater for an evaporator and a superheater, placed in the combustor, for the generated steam (PFBC are the initial letters of the English expression Pressurized Fluidized Bed Combustion).
In a PFBC power plant, an optimum dimensioning of the feed water preheater, the evaporator and the superheater entails special problems at a very low load. It is advantageous to utilize the cooled walls of the combustor for preheating the feed water. These walls may form the entire feed water preheater or a part thereof. At a very low load, the necessary water flow for cooling of the combustor walls may exceed the water requirement in the evaporator of the plant. This means that too small a proportion of the supplied water is evaporated in the evaporator. The steam flow through the superheater may become insufficient so that its boiler tubes reach too high a temperature and are damaged. Upon a load drop out and a GT (gas turbine) trip, the large heat contents in the bed material of the combustor entail special problems. The water flow required for cooling the walls of the combustor is so great that the same flow through a subsequent evaporator results in very little steam being generated and in the tubes of the superheater not receiving a steam flow necessary for the cooling thereof, with an ensuing risk of these tubes being damaged.